In satellite-based positioning systems, commonly referred to as GNSS, the acronym standing for Global Navigation Satellite System, a fixed or mobile satellite terminal such as a vehicle or an aircraft, is located by trilateration (trilateration is a mathematical procedure making it possible to determine the relative position of a point by using the geometry of triangles just like triangulation. But in contradistinction to the latter, which uses angles and distances to position a point, trilateration uses the distances between a minimum of two reference points) by means of the calculation of the distances separating it from several satellites. The precision in the evaluation of the distance between the satellite terminal or the ground station and each of the satellites is determinantal in obtaining precise positioning. The main source of error in the evaluation of this distance is the retardation accumulated by the signal when it passes through the ionosphere, where the partial ionization of the gases at high altitude disturbs the propagation of the signal and brings about a variable transmission delay.
In order to obtain a precise measurement of the positioning, it is therefore necessary to be able to procure an estimation of the time for the journey between the satellites and the ground station or the satellite terminal, as well as an estimation of the journey time estimation error. This is typically a fitted covariance. Accordingly, the known GNSS systems can be supplemented with so-called augmentation systems, which in real time deliver corrections connected with the activity of the ionosphere. These systems also deliver indicators of integrities guaranteeing these corrections. Augmentation systems based on satellites, commonly referred to as SBAS, signifying Satellite-Based Augmentation System, are known. Such is the case, for example, for the EGNOS system (for European Geostationary Navigation Overlay Service) used in Europe, which broadcasts from several geostationary satellites correction data destined for ground stations or satellite terminals using the GPS system (GPS signifying Global Positioning System).
In a known approach, commonly referred to as the TRIN model, the acronym standing for TRiangular INterpolation, the ionosphere is likened to a thin layer around the terrestrial globe, in which the entire electron charge of the ionosphere is accumulated. This thin layer is modeled by a linear model formed of a regular polyhedron, centered on the earth and possessing one thousand two hundred and eighty triangular faces.
Augmentation systems use interpolation procedures to determine the propagation delay of a signal passing through a determined penetration point. A penetration point is a point of the polyhedron situated at the intersection with the sight axis passing through the satellite and the satellite terminal or the ground station. The penetration point is commonly referred to as an IPP, the acronym standing for Ionosphere Pierce Point. The interpolation calculations are carried out on the basis of the knowledge of the retardations of the signals passing through nodes of the polyhedron close to the penetration point considered. In addition to the estimation of this propagation delay, an estimation of the impreciseness in this delay is carried out.
The estimation of the Vertical Total Electron Content, known by the acronym VTEC, makes it possible to establish an ionospheric chart of the retardations in the form of a spherical grid centered on the earth, situated at altitude and tied to the rotation of the earth. The nodes of the grid are dubbed IGP, the acronym standing for Ionospheric Grid Point. The systems known by the name SBAS broadcast to users the vertical ionospheric delays above the points of the grid, commonly referred to as an IONO grid, as well as reliability information to calculate the ionospheric retardation on the lines of sight of each satellite considered, by linear interpolation on this grid. A regular update of the values of the IONO grid, typically every 30 seconds, is broadcast by the systems known by the name SBAS destined for users of the navigation system.
This IONO grid which is accessible to users of navigation systems, is calculated by linear interpolation on the basis of the TRIN model at a fixed solar time, the exposure to the sun of each of the nodes of the polyhedron being constant.
This IONO grid therefore contains the necessary information regarding vertical delays which is required so that a user can reconstruct an estimation of the delay experienced by the signal during its crossing of the ionospheric layer. The method is known and standardized by the MOPS, the acronym standing for Minimum Operational Performance Standard. Knowing the vertical delay GIVD, the acronym standing for Grid Ionospheric Vertical Delay, and associated error GIVE, the acronym standing for Grid Ionospheric Vertical Error, for 4 points of the IGP grids, the user calculates, by linear interpolation the vertical delay at the pierce point IPP of the satellite-user sight axis, as well as the associated error UIVE, the acronym standing for User Ionospheric Vertical Error. Finally the user applies a mapping function (modeled and standardized by the MOPS) to pass from delays and errors vertically plumb with the point IPP to delays and errors along the sight axis at the same point IPP.
However the methods, known in the prior art, for determining this impreciseness are not precise enough to perform an effective check of the satellite-based augmentation system.